This invention relates to a current limiting device for use in electric power transmission and distribution systems.
In electric power transmission and distribution systems, a fault current condition may result from events such as lightning striking a power line, or downed trees or utility poles shorting the power lines to ground. Such events create a surge of current through the electric power grid system (EPGS) that can cause serious damage to the EPGS equipment. Switchgears are deployed within electric distribution and transmission substations to protect substation equipment from such damages. However, due to the continuing growth of power demands and increased inter- and/or intra-connections between power distribution networks, transmission networks, and power generation sources, fault current level could be increasing to a level that exceeds the original fault current interrupting capabilities of the switchgears. Application of current limiters would reduce the available fault current to a safer level where the switchgears can perform their protective function for the EPGS, without resorting to other expensive measures such as replacing all the switchgears or building new substations.
Superconductors, especially high-temperature superconducting (HTS) materials, are well suited for use in a current limiting device because of their intrinsic properties that can be manipulated to achieve the effect of a xe2x80x9cvariable impedancexe2x80x9d under certain operating conditions. A superconductor, when operated within a certain temperature and external magnetic field range (i.e., the xe2x80x9ccritical temperaturexe2x80x9d (TC) and xe2x80x9ccritical magnetic fieldxe2x80x9d (HC) range), exhibits no electrical resistance if the current flowing through it is below a certain threshold (i.e., the xe2x80x9ccritical current levelxe2x80x9d (JC)), and is therefore said to be in a xe2x80x9csuperconducting state.xe2x80x9d
However, if the current exceeds this critical current level the superconductor will undergo a transition from its superconducting state to a xe2x80x9cnormal resistive state.xe2x80x9d This transition of a superconductor from a superconducting state to a normal resistive state is termed xe2x80x9cquenchingxe2x80x9d. Quenching can occur if any one or any combination of the three factors, namely the operating temperature, external magnetic field or current level, exceeds their corresponding critical level.
The surface plot shown in FIG. 1 illustrates the inter-dependency among these three factors (TC, HC, and JC) for a typical superconducting material. As shown in FIG. 1, the surface plot includes three axes T, H, and J, where TC is the critical temperature below which the superconducting material must be cooled to remain in the superconducting state, where HC is the critical magnetic field above which the superconducting material cannot be exposed in order to remain in a superconducting state, and where JC is the critical current density in the superconducting material that cannot be exceeded for the superconductor to remain in a superconducting state.
The xe2x80x9ccritical J-H-T surfacexe2x80x9d represents the outer boundary outside of which the material is not in a superconducting state. Consequently, the volume enclosed by the critical J-H-T surface represents the superconducting region for the superconducting material.
A superconductor, once quenched, can be brought back to its superconducting state by changing the operating environment to within the boundary of its critical current, critical temperature and critical magnetic field range, provided that no thermal or structural damage was done during the quenching of the superconductor. An HTS material can operate near the liquid nitrogen temperature (77K) as compared with a low-temperature superconducting (LTS) material that operates near liquid helium temperature (4K). Manipulating properties of a HTS material is much easier because of its higher and broader operating temperature range.
The quenching of a superconductor to the normal resistive state and subsequent recovery to the superconducting state corresponds to a xe2x80x9cvariable impedancexe2x80x9d effect. A superconducting device with such characteristics is ideal for a current limiting application. Such a device can be designed so that under normal operating conditions, the operating current level is always below the critical current level of the superconductors, therefore no power loss (I2R loss) will result during the process. When the fault occurs the fault current level exceeds the critical current level of the superconducting device thus creating a quenching condition. By the same token, mechanisms altering the device operating temperature and/or magnetic field level can be put in place either as a catalyst or an assistant to achieving fast quenching and recovery of such a superconducting device.
McDougall, et al., U.S. Pat. No. 6,043,731, entitled xe2x80x9cCurrent Limiting Device,xe2x80x9d describes a superconductor device that uses an active control mechanism to adjust the critical current level of a superconductor. Under the normal operating condition, a magnetic field is applied to the superconductor, causing its critical current level to be less than the maximum. An active control scheme is in place to adjust the critical current density of the superconductor under the fault condition so that its critical current level is below the fault current level, triggering the quenching of the superconductor, thus introducing the current limiting impedance into the circuit it is connected to. After the fault current is limited, this control mechanism is used to increase the critical current level of the superconductor causing the superconductor to return to its superconducting state. A drawback of the current limiting device of McDougall, et al. it that it requires an active control scheme incorporating an external power supply source to achieve the effect of xe2x80x9cadjustable impedance,xe2x80x9d which increases the complexity and cost of the design and raises reliability issues.
Saravolac, U.S. Pat. No. 6,137,388, dated Oct. 24, 2000 and entitled xe2x80x9cResistive Superconducting Current Limiter,xe2x80x9d describes a superconductor that is placed inside a nonmetallic cryostat filled with a cooling medium to maintain the superconductor in a superconductive state. A foil winding is connected in series with the superconductor by current leads and the cryostat is placed inside the winding. Under normal operating conditions, the current in the foil winding generates a persistent magnetic field that is parallel to the superconductor, with the current below the critical current level and the persistent magnetic field below the critical magnetic field of the superconductor. In the event of a fault, the current in the foil winding increases to a level that generates a magnetic field that exceeds the critical magnetic field of the superconductor and triggers the superconductor to a resistive state. This invention does achieve passive triggering of the superconductor quenching. A drawback of Saravolac""s current limiting device of it that the foil winding that provides trigger magnetic field during a fault also puts the superconductor in a persistent magnetic field under normal operating mode. This persistent magnetic field is sufficient enough to degrade the superconductor""s performance. Furthermore, it would be very difficult to locate superconducting materials in the uniformed magnetic field region within such a device to reduce mechanical stress exerted by the Lorentz force (i.e., Force (F) acting on a moving particle with charge q and velocity v in a magnetic field B, where F=q vxc3x97B). In addition, there will always be a voltage drop across this device because of the inductive nature of the foil windings and substantial I2R loss associated with such a design.
It is therefore an object of this invention to provide a current limiter that, under normal operating condition, will pass current through path(s) composed of only superconducting components that are not under any influence of an external magnetic field.
It is another object of this invention to provide a current limiter that detects and limits fault current, and subsequently recovers to its superconducting state automatically without resort to incorporating any kind of active switching and controlling mechanism.
It is yet another object of this invention to provide a current limiter that is composed of easily acquired modular components that enable scalability for a range of applications and operating scenario as are used in varieties of electric power distribution and transmission networks.
It is yet another object of this invention to provide a current limiter that is highly reliable with built in redundancy in the design such that a failure of any individual component does not result in a failure of the entire device.
The present invention is a current limiting device incorporating components made of superconducting and non-superconducting electrically conductive materials. This so-called Matrix-type Fault Current Limiter (MFCL) device includes a trigger matrix having xe2x80x9c1xc3x97nxe2x80x9d (columnxc3x97row) number of trigger elements electrically connected in series with a current limiting matrix containing xe2x80x9cmxc3x97nxe2x80x9d number of current-limiting elements. Each trigger element within the trigger matrix includes one non-inductively arranged superconducting component electrically connected in parallel with a non-superconducting inductor group containing xe2x80x9c1+mxe2x80x9d number of parallel connected inductors. Each current limiting element within the current limiting matrix includes one non-inductively arranged superconducting component electrically connected in parallel with one non-superconducting inductor.
The xe2x80x9c1+mxe2x80x9d number of inductors in the inductor group of a trigger element are physically wound around the respective superconducting component in the xe2x80x9c1xe2x80x9d trigger element plus xe2x80x9cmxe2x80x9d number of current limiting elements that have the same row number in both matrices.
The number of rows xe2x80x9cnxe2x80x9d in the matrices is determined by the peak normal operating current level (with consideration given to any normal fluctuation of such a current level) that passes through the MFCL of the present invention. This peak current level (plus whatever fluctuation level to be included), divided by the number of rows in the MFCL matrices, at minimum, should not exceed the critical current level of each individual superconducting component used in the MFCL matrices (assuming the device uses only identical superconducting elements). More rows can be added to increase the redundancy of the design and therefore overall reliability of the MFCL device. The number of columns xe2x80x9cmxe2x80x9d in the current limiting matrix is primarily determined by the current limiting impedance required for a specific electric network, making an MFCL design highly scalable.
Under the normal operating condition, the current passes through only the non-inductively arranged superconducting components within the MFCL device, thus producing no voltage drop across the device or I2R loss due to the zero electrical resistance nature of the superconducting materials. However, when a fault condition occurs the surged current in the electric network exceeds the critical current level of the superconducting components, creating a transition from a superconducting state to a normal resistive state.
Such a transition creates a current sharing regime between the superconducting component and the inductor group in a trigger matrix element, and between the superconducting component and the inductor in a current limiting matrix element. The diverted current in each of the xe2x80x9c1+mxe2x80x9d inductors within a trigger matrix element will in turn generate a substantial magnetic field that surrounds each superconducting component in the xe2x80x9c1xe2x80x9d trigger matrix element and xe2x80x9cmxe2x80x9d number of current limiting matrix elements. This magnetic field is designed to exceed the critical magnetic field level of the superconducting components in the device, therefore further speed up superconductors"" transition from the superconducting state to the resistive state, thus introducing the necessary current-limiting impedance of the MFCL device into the electric network.
All three factors, namely the current surge that exceeds the critical current level of superconducting components, the superconductor temperature rise associated with heating by the excessive current, and the external magnetic field generated by the current sharing regime, work to promote the transition of superconductors in the MFCL from the superconducting state to the resistive state. The parallel-connected inductors in both the trigger and current limiting matrices serve to protect the superconducting components from a transient voltage surge that is usually associated with the rapid increase of the fault current level. The partial divergence of the surged current to the inductors also serves to reduce the thermal energy that the superconductors must absorb during the current limiting phase of the MFCL operation. This makes fast recovery of an MFCL device to its superconducting state more attainable.